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Research Papers

Spherical Hands: Toward Underactuated, In-Hand Manipulation Invariant to Object Size and Grasp Location

[+] Author and Article Information
Raymond R. Ma

Department of Mechanical Engineering and Materials Science,
Yale University,
9 Hillhouse Avenue,
New Haven, CT 06511
e-mail: raymond.ma@yale.edu

Nicolas Rojas

Department of Engineering and Design,
School of Engineering and Informatics,
University of Sussex,
Brighton BN1 9QT, UK

Aaron M. Dollar

Department of Mechanical Engineering and Materials Science,
Yale University,
15 Prospect Street,
New Haven, CT 06520
e-mail: aaron.dollar@yale.edu

Manuscript received March 24, 2016; final manuscript received September 5, 2016; published online October 25, 2016. Assoc. Editor: Leila Notash.

J. Mechanisms Robotics 8(6), 061021 (Oct 25, 2016) (12 pages) Paper No: JMR-16-1080; doi: 10.1115/1.4034787 History: Received March 24, 2016; Revised September 05, 2016

Minimalist, underactuated hand designs can be modified to produce useful, dexterous, in-hand capabilities without sacrificing their passive adaptability in power grasping. Incorporating insight from studies in parallel mechanisms, we implement and investigate the “spherical hand” morphologies: novel, hand topologies with two fingers configured such that the instantaneous screw axes, describing the displacement of the grasped object, always intersect at the same point relative to the palm. This produces the same instantaneous motion about a common point for any object geometry in a stable grasp. Various rotary fingertip designs are also implemented to help maintain stable contact conditions and minimize slip, in order to prove the feasibility of this design in physical hand implementations. The achievable precision manipulation workspaces of the proposed morphologies are evaluated and compared to prior human manipulation data as well as manipulation results with traditional three-finger hand topologies. Experiments suggest that the spherical hands' design modifications can make the system's passive reconfiguration more easily predictable, providing insight into the expected object workspace while minimizing the dependence on accurate object and contact modeling. We believe that this design can significantly reduce the complexity of planning and executing dexterous manipulation movements in unstructured environments with underactuated hands.

Copyright © 2016 by ASME
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Figures

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Fig. 1

Spherical hands are hand topologies incorporating curved fingers with out-of-plane angular offsets designed such that the grasped-object motion is about a common point N, regardless of contact location or system configuration

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Fig. 2

Multiple views of the kinematic structure of the proposed spherical hand, with a traditional two-link thumb and nonpivoting base. The axes of rotation for the curved fingers intersect at a common point regardless of the hand configuration.

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Fig. 3

The graph of kinematic constraints of the hand-object system for the spherical hand (a) and its corresponding reduction ((b) and (c))

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Fig. 4

Structure of thumb designs of the spherical hands: (a) two-link thumb with static base, (b) two-link thumb with pivot base, (c) one-link thumb with pivot base

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Fig. 5

(a) Proposed design of the prototype curved fingers, and (b) physical comparison of the curved fingers from the spherical hand designs with the standard, planar fingers used in traditional hand designs. Other finger link geometries are possible as long as the joint axes' intersection is maintained.

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Fig. 6

The passive, pivoting degree of freedom is implemented such that it is not actuated by the main drive tendon. The drive tendon passes through the rotational axis of the pivot. Extension springs on both sides of the finger base set the initial configuration at center.

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Fig. 7

Manual, unactuated exploration of the reachable kinematic spaces for the standard (b) and (c) spherical hand designs was explored for both two-finger and three-finger contact conditions and a variety of object sizes (a)

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Fig. 8

Set of discrete base configurations that were tested for both the standard and spherical hand fingers. Configuration C is considered to be the ideal spherical hand case, where the joint axes all intersect at a common point.

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Fig. 9

Experimentally sampled workspace projections for the standard hand, for base configuration C and test object size 58 mm, utilizing an ideal kinematic setup with magnetic spherical joints

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Fig. 10

Experimentally sampled workspace projections for the spherical hand, for base configuration C and test object size 58 mm. The spherical surface fitting is more consistent for the spherical hand configuration than traditional hand designs.

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Fig. 11

For a constant tendon actuation length, the passive reconfiguration of the underactuated finger from its free swing trajectory (fCi) to its contact location on the object (Ci) determines the passive set of forces exerted onto the object

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Fig. 12

The rotary fingertips were constructed of a monolithic, cast urethane shell, a neodymium sphere bonded to an M3 bolt, a nylon countersunk washer, and a neodymium disk embedded in the fingertip

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Fig. 13

Multiple fingertip designs were evaluated: (a) rotary icosahedron (I), (b) rotary disk (D), (c) rotary round (R), and static round (SR)

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Fig. 14

Test setup for experimental workspace evaluation. The hand is held upside down in a fixture for test grasps and manipulation motions.

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Fig. 15

The design progression from the (a) OpenHand Model O, to the (b) standard fingers with specialized fingertips, to the (c) spherical hand layout, in this case with a two-link thumb of static base

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Fig. 16

Experimental manipulation workspace for the spherical hand utilizing rotary round fingertips and a thumb base with pivot providing the additional passive degree of freedom. The light gray overlay shows the calculated alpha shape for the respective projection.

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